As for the settings on the h/u... I always, always, always leave mine on "flat", but I also have an amplifier for my mids/highs. Set your subwoofer level on the headunit to the highest you'll ever have it... honestly, leaving it at 0 (if it goes down into the negative numbers) will work just fine. Some headunits don't put out any decent voltage from their sub pre-outs until the subwoofer level is up though... in your case, I'm honestly not sure, so I'd use it with it at 0.